Using organ culture methods, the proposed studies are designed to systematically evaluate the effects of putative transmitters as well as their pharmacological analogues and antagonists on the rhythm of vasopressin release expressed by the circadian clock located in suprachiasmatic explants. Two series of experiments are proposed in which different aspects of the neurochemical organization of the suprachiasmatic nucleus (SCN) from a functional perspective. In the first project, experiments will examine the functional roles of afferent transmitter systems in the central control of the SCN and its circadian output. Emphasis in these experiments has been placed on those putative transmitters thought to be associated with major neural inputs to the SCN, namely acetylcholine, neuropeptide Y and serotonin. In contrast, a second project will approach the functional organization of transmitters systems underlying intranuclear communication in the SCN. Although the inherent cellular neurochemistry of the SCN is heterogeneous in nature, this series of experiments will focus exclusively on the potential role of GABA as an intrinsic timekeeping component involved in mediating the generation and expression of the circadian rhythms by the SCN. The first approach utilized in both of these projects entails an examination of the effects of the transmitters and/or their related analogues and antagonists on an easily measured pacemaker property, the phase of the vasopressin rhythm. This approach will be extended in subsequent studies to: 1) examine the relationship between dose and the phase-shifting action of the agent; 2) determine whether specific antagonists of its pharmacological action can block phase shifts induced by this agent; and 3) systematically examine the phase-shifting responses to acute drug administration at various times throughout the entire circadian cycle. Since the phase of an overt rhythm is thought to directly reflect the state of the circadian clock, the observation that acute drug administration induces phase shifts in the vasopressin rhythm would implicate the related transmitter in the regulation of cellular components of the SCN involved in generating circadian rhythms. Furthermore, the finding that the phase-shifting action of an agent is dependent on the time of administration would indicate that the transmitter plays a role in mediating the entrainment of the SCN and its circadian output. In view of the fact that many physiological systems are influenced by circadian neural signals, the proposed studies, which provide an opportunity to determine whether specific drugs can directly alter a circadian clock, may provide to be useful in the treatment, diagnosis and understanding of physical and metal illnesses that have been associated with a disorder in circadian timekeeping.